An electronic imaging system typically produces a signal output corresponding to a viewed object by spatially sampling an image of the object in a regular pattern with an array of photosensitive elements, such as, for example, with a charge-coupled device (CCD) solid-state image sensor. In such an imaging system, it is well-known that detail components in the object which contain frequencies too high to be analyzed within the sampling interval of the sensor contribute to the amplitudes of lower frequency components, and thereby produce imaging errors commonly referred to as aliasing or undersampling artifacts. In particular, if spatial detail being imaged contains a high frequency component of a periodicity smaller than the pitch (periodicity) of each neighboring photosensitive picture element of the solid state image sensor, the subsequent detection of this high frequency component tends to result in a spurious signal due to aliasing.
In general, the electronic imaging system can minimize aliasing if its optical section has a frequency response that cuts off, or filters out, the higher frequency content of the object. As a result, the optical section generally employs an optical low pass filter to substantially reduce the high frequency component contained in the spatial detail of the image received by the image sensor. It is thus well-known in the prior art that the design of electronic imaging systems involves a trade-off between image sharpness and the susceptibility of the imaging system to aliasing distortions or undersampling artifacts.
To limit these artifacts, an optical filter such as, for example, a birefringent blur filter has become a common component in consumer color video cameras. U.S. Pat. Nos. 4,101,929 and 4,896,217 show typical examples of such filters. Such a filter is typically placed between a lens and the image sensor to provide a low-pass filter function which reduces the spatial frequency content of the object at frequencies above the Nyquist frequency of the photosensitive elements. This makes the imaging system less susceptible to aliasing distortion. For example, for many available sensors wherein equal pixel densities in each of the sensed colors provide that each of the sensed colors have the same Nyquist frequency, an achromatic low-pass, or "blur", function is effective in minimizing aliasing distortion. Such a function can readily be provided by a birefringent filter.
The birefringement blur filter is typically composed of filter plates manufactured from a crystalline material like quartz that exhibits a dual refraction effect when the crystal axes of the filter plates are oriented at an angle with respect to the plate surface. In this orientation, a randomly polarized ray of light passing through such a filter plate emerges as two separated polarized rays. The combination of several such plates produces a multiple spot pattern from each incident point in the image. If this spot pattern distributes light energy over multiple photosensitive elements, then the effect of a blur is obtained. This will limit the optical transfer function of the system at spatial frequencies above the Nyquist frequency of the photosensitive elements. However, this type of filter suffers from the drawback that it is costly and complicated to manufacture. In addition, a practical birefringent filter tends to be rather large and thick. Indeed, the thickness required to achieve the desired blur requires a lens with a long back focal length in order to make room for the blur filter in the optical path. Space limitations often do not allow such an optical structure, and lens design becomes unduly complicated. Finally, since such a filter requires randomly polarized, or non-polarized, light, a polarizing filter cannot be allowed in such a system to obtain well known photographic polarizing effects.
As can be appreciated from the foregoing remarks, there is a need in the art for a physically small blur filter that is inexpensive and relatively simple to manufacture, yet produces a tightly controlled blur pattern that is not dependent upon polarization techniques. As an alternative to the birefringent blur filter, copending Ser. No. 88/040,713, entitled "An Optical Fiber Filter for Reducing Artifacts in Imaging Apparatus" and filed on even date herewith by common assignee, describes the use of an array of optical fibers as a blur filter to reduce undersampling artifacts in an imaging system. Such an imaging system is shown in FIG. 1, in which an image of an object 1 is generated by a lens 2 and directed through an optical fiber array 3 upon an image sensor 4, which is composed of a two-dimensional array of photosites 5. The optical fiber array 3 is arranged in a two-dimensional structure of straight optical fibers 6 such that each fiber receives image light over a discrete image pixel area and integrates the light by multiple reflections, thereby removing high frequency content within the pixel area. In addition, by separating the optical fiber array 3 from the sensor 4 (by a distance d), each fiber 6 emits an exit cone of light 7 over adjacent photosites 5. The combined effect of light integration and the cone-shaped spread of light exiting each fiber can be controlled (by varying the distance d) to provide a controlled blur pattern on the photosites 5, which blocks higher spatial frequencies from reaching the image sensor 4.
While an optical fiber blur filter is simpler and less complicated to manufacture than a birefringement filter, and while it produces a blur pattern that is independent of the state of polarization, it has a drawback in that the angle of the exit cone 7 is dependent upon the optical aperture of the optical system. This happens because the exit cone angle of light from each fiber is equal to the angle of the entrance cone. In an optical system with a variable aperture, the entrance cone angle will vary with the size of the aperture. This is shown schematically in FIGS. 2A and 2B for two entrance cones, set by respective positions of a diaphragm 8, to one optical fiber 6. For typical camera lenses, the aperture can vary between f/numbers of 2.8 and 32, and the cone angle of light introduced into the fibers will vary between 10.1.degree. and 0.89.degree., respectively. This provides the angle variation in exit cone 7a and 7b seen in FIG. 2A and 2B, respectively. The blur spot size over the photosites 20 therefore will vary almost 10 to 1 at the extremes of a typical aperture adjustment. This is shown for a configuration of four neighboring photosites in FIGS. 3A and 3B. A small blur spot 9a in FIG. 3A corresponds to the image coverage for a small aperture (FIG. 2A) and a large blur spot 9b in FIG. 3B corresponds to the image coverage for a large aperture (FIG. 2B).
The problem is clear from an inspection of FIGS. 3A and 3B. The amount of blur is highly dependent upon the aperture, and can disappear altogether if the smaller blur spot 9a is wholly within one photosite. Moreover, to maintain a constant blur spot diameter, the spacing d between the sensor 4 and the optical fiber array 3 would have to vary with the aperture. This is mechanically difficult to accomplish.